May 3, 2016

6J5 Line preamplifier Part 2

High voltage requirements

Needless to say that a good supply, free of ripple, is essential in SE stage that have very bad CMRR. This generally dictate a large capacitor bank at the expense of transient response due to great time recovery.
As I prefer a punchy response it needs a good calculation of the filtering cells to keep ripple at least 10-5 (- 80 dB) below last stage voltage. Means 240*10-5 or 2,4 mV rms while keeping the smallest possible time constant.
Using a input choke of large value greatly helps reducing the following capacitors value. Furthermore behind the choke the ripple is very smooth and quite sinusoidal, generally 2 cells are enough to have an almost perfect continuous voltage.

Main supply



To calculate the ripple behind the choke we have to know the ripple factor   η = 1,2/L*C
 (L in Henries and C in µF) where C is the first filtering cap.
An estimation of this cap can be done if we consider a time constant not more than 15 millisecond. The choke having a DC resistance of 800 ohm, C will be 15 10-3/800 = 18,7 µF, the closest usual value is 16µF.
Then η = 0,00125 and the ripple on the first cap will be ηV = 0,00125*265 = 0,33Vpp or 0,117 Vrms. Network time constant is 13 millisecond.
A second cell with 1,2 Kohm and 16µF will leave a small 25mVpp / 8.8mVrms ripple with a 19 millisecond TC.
A last cell with 2,4 Kohm and 10 µF will leave 1,6 mVpp / 0,56mVrms and a 24 millisecond TC.
These 0,56mV AC represents a -113dB attenuation below the 240V feeding the 6J5, way better than the -80dB expected, while keeping a very fast recovery power supply. I did some tests with pulse trains and supply is rock solid.

* Time constant is a very important factor in audio, it determines the capacity of an RC system to respond to an AC signal modification and is essential in the ability to follow the sound envelope or ADSR. It is too often neglected when calculating a power supply.
In the case of AC circuits, the RC time constant tells you which signals the circuit passes through and which signals the circuit filters out. An RC circuit acts as a high pass filter which passes high frequency signals and blocks low frequency signals.
When we apply an AC current to a capacitor we are basically sticking some charge onto the capacitor and then taking it off. If we do this at a low rate, smaller than the time constant, then the capacitor has time to discharge. But if we put the charge there rapidly, at a speed much greater than the time constant, the capacitor doesn't have time to discharge. In this way, we can pass high frequency signals through the capacitor.
Remember :
-1 A given capacitor charges 99,9 % at 5 RC.
-2 As a rule of thumb, TC should increase from the first to the last decoupling cell.


No more technical considerations...

Construction

Like for the line preamp just a few parts but of high quality.
An RCore transformer for low overall magnetic loss, high efficiency (just bettered by the toroidal) and slim design to fit my chassis.

A rectifier, good capacitors (German F&T dual 16µf and a couple of Siemens 8µF MP/JS + 2µF Sprague PIO to make my 10µF last decoupling cap), a few resistors, a filtering choke and quality sockets.
Add some hardware and plenty of time....and you are done.


In colour with Neotron 6J5G




For chassis I used 2U professional grade steel ones, 3 layers blue gray paint, very well suited for tubes with lots of openings for optimal air flow. They provide good shielding and are super rugged. I just add some screen printed front and rear plates for better appearance and way better WAF.




Listening report

The 6J5 and its relatives are too numerous for an extensive test. I will give my impress upon the most interesting ones I have in stock. Anyone wanting to build this simple BUT "great performer on any kind of music" preamp will easily find a wide range of readily available tubes.
All the tubes listed below are up to the task, some are more involving or more waaahou !! on a specific material, but a very few simply made me listen to the music instead of searching the preamp qualities and / or shortcomings.
Most need a few burning hours to deliver there full colour.

6J5V Radio Technique
Best of all. Just say evident whatever the music. Delightful triode, vivid, natural with lots of air, detailed to the extreme but never tiring. This is the 6J5 "just sit and listen". These PTT tubes were intended for very long life service, each tube came with a numbered biographical sheet to keep records for 15 years!

6J5G Neotron
Would be my second best tube. A bit more brilliant with slight grain on voices, still stays very detailed and involving. Due to incestuous relationships between tube makers Neotron's were possibely made by Visseaux (with slight construction differences). I have some tubes bearing both Visseaux and NT labels. So who made what ?

L63 / CV1067 GEC Straight Bulb
Nice sound, would say polite, lacks the details of the RT's or Neotron's, no grain but a smaller sound stage that make music more intimate.
By comparison the L63 GEC ST (see part 1) have a bigger and wider sound but are surprisingly very slow tubes that give the feeling of wrong tempo.

6J5WGT Raytheon
I did not like much this tube despite some remarkable ability on voices. Muddy sound with grain, tight and punchy bass but not as accurate as the other ones. Huge sound stage, lush presentation BUT if you like Billy, Ella and Louis or country singers with hoarse voices, this tube gonna make you cry.

Last of the batch 7193 National Union
I had this tube for decades and it is the kind of weird thing you don't want to exhibit. It is a mistake, and I regret not having foreseen the holes for the anode / grid wires. This tube is a killer by many aspects, it has the CV1067 GEC qualities with a much more authoritative presentation and a wide but realistic sound stage.

I was willing to test the 6P5G that bear an excellent reputation, unfortunately impossible to source a NOS pair at a decent price.

Like in every tube based device the rectifier plays an important role in final restitution.
Here, due to the very few parts used its impact is more than evident and the choice made will determine the preamp character. I tried all the ones that could fit, directly or indirectly heated. On the dozen I listened to, the very best for neutral but vivid presentation are Mullard GZ30/CV2748, Mazda GZ32 and STC/ Brimar 5Z4G, with a slight margin for the GZ30 in terms of bass resolution and voices transparency.




3 comments:

  1. Hi!
    May I ask what is this beautiful volume changing knobs and where can they be found?
    Thanks a lot,
    Regards,
    Maxime

    ReplyDelete
    Replies
    1. Hi Maxime, it's part of the front plate and custom made.
      Best regards.
      Fred

      Delete